1. Field of the Invention
The present invention relates to ball grid array semiconductor packages and methods of attaching and interconnecting the same. In particular, the present invention relates to interposers for mounting a semiconductor die to a substrate with which the semiconductor die is in electrical communication The semiconductor die may be encapsulated upon the interposer to form a complete semiconductor die package. The interposer may be constructed in order to allow a number of similar semiconductor die packages to be electrically connected to a single substrate in a stack.
2. State of the Art
Ball grid array (“BGA”) packages are well known in the art. BGA packages typically comprise a substrate, such as a printed circuit board, with a semiconductor die mounted on top of the substrate. The semiconductor die has a multitude of bond pads electrically connected to a series of metal traces on the top side of the printed circuit board. This series of metal traces is connected to a second series of metal traces located on the underside of the printed circuit board by a series of vias. The second series of metal traces each terminate with a contact pad where a conductive element is attached. The conductive elements are typically solder balls or conductive-filled epoxy. The conductive elements are arranged in an array pattern and the semiconductor die along with its electrical connections is encapsulated with a molding compound.
As die and grid array densities increase, the desire in packaging semiconductor dies has been to reduce the overall size of the package, allowing the mounted packages to take up less “real estate” or area within a device. A BGA package mounted in a flip chip fashion conserves real estate laterally by eliminating the connection structures (wirebond, TAB, etc.) around the package. Conserving real estate vertically presents different challenges. One way this has been accomplished is by reducing the overall height or profile of BGA packages. Another way is by using molded packages with leadframes that can be stacked atop one another.
Stacking BGA packages allows additional lateral real estate to be conserved in exchange for vertical space. The height of a BGA package, including the substrate, semiconductor die, wirebonds, encapsulant and conductive elements, limits the effectiveness of this method. One example of an attempt to reduce the height of BGA packages in a stack is disclosed in U.S. Pat. No. 6,072,233 issued to Corisis et al., Jun. 6, 2000. The disclosed packages consist of a semiconductor die mounted face down on a thin substrate. The die is electrically connected to the substrate by centrally located wirebonds, which are encapsulated on the base of the substrate. Solder balls which have a height greater than the combined heights of the mounted semiconductor die and the wirebond encapsulant are spaced along the perimeter of the substrate. By moving the encapsulant to the lower surface of the package and using a thin substrate, a fine ball grid array (FBGA) can be used, reducing the overall height of a package. The ability to reduce the height of the package is still limited by the placement and size restrictions on the solder balls, which must have a diameter greater than the combined heights of the semiconductor dies and wirebond encapsulant. The substrate must be rigid to hold the individual packages apart. The solder balls are exposed at the side of the stack, increasing the chances of breakage or contamination.
Therefore, it would be advantageous to develop an apparatus and method that allow for the height of stacked packages to be reduced in comparison to conventional packages. It would further be advantageous for such an apparatus and method to provide for at least enclosing the conductive elements, providing enhanced support as well as protection from breakage and contamination. It would be an additional advantage for certain embodiments of such an apparatus and method to employ at least somewhat flexible substrates, allowing use in an increased number of applications.